1. Field of the Invention
This invention relates in general to the field of battery charging devices, and more particularly to those battery charging devices that also supply power to additional electrical loads.
2. Description of the Related Art
Portable computers, personal digital assistants (PDA's), cellular telephones, pagers, calculators, and other such electronic devices are commonplace in today's mobile society. One of the reasons portable electronic devices are so popular is that they provide a user with virtual freedom regarding the location of their use, as long as a source of power is readily available. Although these devices may be powered by plugging them into a standard AC outlet, AC power is often not convenient or readily available. Hence, their real portability and utility comes from their being powered by batteries.
Early portable devices relied on rechargeable lead-acid batteries to provide them with power. However, for some devices such as portable computers, the power requirements were so great that the "on-time" of the device, i.e., the useful battery life between charges, was often measured in minutes, rather than hours. In addition, early rechargeable batteries were inefficient at recharging, having recharge times three to four times that of their useful life. For example, one early portable computer manufactured by the assignee of the present invention had an on-time of approximately 45 to 100 minutes, with a recharge time of approximately 6 to 8 hours. For many users, this amounted to an overnight charge to obtain an hour or so of useful life during the day. What was needed, and what the market has continually demanded, is longer lasting (higher watt-hour) batteries, as well as batteries having faster recharge times.
Although there have been tremendous improvements in battery technology, other factors important to the marketplace have developed. Users not only want longer life, with shorter recharge times, they also want to be able to charge their batteries while still using their portable device (for faster recharging), and they want to carry as few items as possible, having as little weight as possible, when traveling with their portable device. Another example will help illustrate this point.
Early portable computers operated from an array of rechargeable lead acid batteries which provided portable power. These arrays, or battery packs, weighed between 1.5 and 3 pounds. To recharge a battery pack, a battery charger was required which converted AC power into DC power appropriate for recharging the battery pack. The battery pack was inserted into the charger for charging, and later removed and placed into the portable device for use. This charger was also a bulky device which the user was required to carry. In addition, an AC adapter was also required which could provide power to the portable computer when the battery pack was recharging. Thus, a "portable" computer user, while traveling, was required to carry: 1) the computer; 2) the rechargeable battery pack; 3) the recharger; and 4) the AC adapter. This combination was bulky and cumbersome. The marketplace has therefore demanded, in addition to improved battery technology as referenced above, a system for recharging that is more manageable.
A first solution to the above stated problems was to move the electronics required for charging the battery pack into the portable device. With this solution, the AC adapter is now used to provide DC power to the portable device, both for operation of the device, and for recharging of the battery pack. But, as the power requirements of the portable device increased, the size and cost of the AC adapter also increased. To keep the cost and size of the AC adapter to a minimum, the AC adapter is typically designed to provide just enough power to operate the portable device, but no more. With this arrangement, the AC adapter provides power to the portable device while the portable device is being used, but no additional power is provided to the battery pack for charging. When the portable device is turned off, charging of the battery pack begins, or resumes. This solution has been chosen by many portable computer manufacturers because it allows the least expensive and smallest AC adapters to be used.
However, this solution does not allow the battery pack to be charged while the portable device is in use. This results in longer charge cycles for those instances when a user requires operation of the portable device. A solution to this problem is to provide an AC adapter that is capable of providing the maximum power required by the portable device, plus that power required for recharging the battery pack. This allows simultaneous use of the portable device, and recharging of the battery pack. But this solution comes with a price: while allowing a user maximum versatility, with minimum charge time, the AC adapter increases both in size and expense.
A compromise in the above two solutions is to provide an AC adapter with a small amount of overhead which can power a portable device, and simultaneously supply a small amount of charge current to the battery pack. When the portable device is being used, the battery pack is charged with one fixed current. When the portable device is not being used, i.e., is turned off, the battery pack is charged with a second higher current. This two charge rate compromise allows a smaller, less costly AC adapter than mentioned above, yet still allows the battery pack to be charged while the portable device is on. Yet, the compromise provides for only two rates of charging. Such a two rate system results in excessive charge times, or oversized AC adapters. In addition, such a fixed rate system fails to recognize that the portable device does not always require maximum power when in operation.
For the foregoing reasons, there is a need for a high performance adaptive power battery charging apparatus that will allow simultaneous charging of a battery pack and operation of a portable device, that takes advantage of the variance in power requirements of the portable device and adaptively charges the battery pack with the remaining power supplied by the AC adapter.